Nickel base alloy



Nov. 3, 1964- M. KAUFMAN ErAL 3,155,501

NICKEL BASE ALLOY Filed June 30, 1961 2 Sheets-Sheet 1 #rivela/Ey*-Novi-3, 1964 M. KAUFMAN ETAL 3,155,501

NICKEL BASE ALLOY Filed June 30, 1961 2 Sheets-Sheet 2 'Tris UnitedStates Patent 3,155,501 NICKEL BASE ALLOY Murray Kaufman, West Peabody,and Robert Francis Wilde, Lynnfield Centre, Mass., assignors to GeneralElectric Company, a corporation of New York Filed .lune 3l), 1961, Ser.No. 121,144

3 Claims. (Cl. 75-171) This invention relates to multiphase nickel basealloys, particularly to one having a unique combination of greatlyimproved strength below about 1700D F., coupled with high strength at1700-1800 F.

As the Iart of metallurgy has improved, a more complete realizationv ofthe interdependence of elements and the phases they form has becomeknown. In the development of nickel base alloys, sometimes classed assuper alloys, the elements Ti, Al, Co, Cr, C and Mo have all been addedto Ni to form a nickel Ibase alloy. Sometimes elements such as Fe, Cb,Zr, B or W have been used as alloying additions.

In many reported alloying situations and combination of elements,different relationships exist between the elements. Different phases ofdifferent physical and mechanical characteristics, solution temperaturesand melting points are formed. Nevertheless, despite the large amount ofdata available concerning nickel base alloys, it is still not possiblefor a metallurgist to predict accurately the physical and mechanicalcharacteristics of each and every phase formed from every givencombination of elements. Thus unexpected results can still be obtainedeven in an area so widely studied.

It is a principal object of this invention to provide a nickel basealloy having a careful and critical balance of alloying elements toresult in an alloy of improved strength below about l700 F. coupled withstrength in the 1700-1800 F. range at least equivalent to known alloys.

These and other objects and advantages will be recognized from thefollowing detailed description, the appended claims and the drawing inwhich:

FIG. 1 is a graphical comparison of some tensile strength properties ofthe alloy of this invention, designated as S15, with the strongestcommercially available alloys;

FIG. 2 is a graphical comparison of the 0.2% yield strength propertiesof the alloy of this invention, designated as S-IS, with the strongestcommercially lavailable alloys; and

FIG. 3 is a graphical comparison of stress rupture properties of thealloy of this invention, designated as S15, with the strongestcommercially available alloys.

The alloy of this invention, designated as S- in the drawing andsometimes referred to as SEL l5, in one form, consists by weight of 4-7%Al, 1.5-4% Ti, 7-16% Co, 8-16% Cr, 5.5-8% Mo, 0.03-0.l% C, 0.3-l.2% Cb,0.003-0.4% B, l-3% W with the balance Ni and incidental impuritiesincluding Fe, Mn, Si etc. up to about 2% maximum and with the furtherprovision that the minimum nickel percentage must be 4-i-6 (percentAl-I-percent Ti) when the Ai/Ti ratio is greater than 2 and 4+5 (percentAl-l-percent Ti) when the Al/Ti ratio is lower than 2. In some forms upto 0.2% Zr can be included.

The alloying elements and their ranges have been carefully selected toprovide an improved a-lloy of unexpected strength propertiesparticularly in the range below 1700 F. while maintaining a highstrength level between 1700- 1800 F.

As is shown by the above formulae for the minimum weight percent Ni, aparticularly critical relationship exists between the elements Ni, Aland Ti. This relationship coupled with carbon and the carbide formersincluded sassari Patented Nov. 3, 1964 ICC within the range of thisalloy are particularly signicant in providing the unexpected result.

In order to obtain maximum strength characteristics in a nickel basealloy including the elements Ti and Al, it would ybe desirable toinclude as much Al and Ti as possible to form Ni3(Al, Ti). However thereis a composition limitation. In order to obtain high temperaturestrength between 1700-1800 F. without regard to other properties inother ranges, the Al ordinarily should be about 2 to 31/2 times the Ticontent. It the Ti is increased at the expense of Al, lower temperaturestrength is gained but at a loss of higher temperature strength. Thiswas recognized by notingvthat the solution temperature of Ni3(Al, Ti)was higher with higher Al/Ti ratios. Also it has been recognized that ifthe Ti content is below 1.5%, lower temperature strength is lost. Forexample, referring to FIGS. l and 2, the alloy indicated as Alloy S-15is the alloy of this invention and Alloy A is a nickel base alloyconsisting, by weight, of 5.5-6.5% Al, (12S-1.25% Ti, 1l-l4% Cr,'i5-5.5% Mo, 0.0=5-0.20% C, l-3% Cb, 0.05-0.2% Zr, 0.005-0.02% B withthe balance essentially nickel. Note that the Al/Ti ratio is high. lt isalso to be noted how the low temperature strength is less than the alloyof this invention. The same reduction in properties can be seen in lthestress rupture comparison of FIG. 3. Both effects are due to the highAl/ Ti ratios in Alloy A.

In FIG. 3, the stress rupture strengths are represented by thecomparison of stress with a time-temperature parameter shown at thehorizontal coordinate. This parameter, known as the Larson-MillerParameter, has been calculated from the formula P=T (20-Hog t) -il03 inwhich P equals the time-temperature parameter, T equals absolutetemperature in degrees Rankine and t equals the time in hours.

Therefore in order to obtain the same or better properties at 1700-1800"F. and greatly improve the strength properties at 1700 F. or below, ithas been found that an optimum amount of Ni3(Al, Ti) is required in thecareful and critical balance of the elements of the alloy of thisinvention. The formulae listed above are important features of thisinvention in view of the fact that the Ti level is maintained within therange of 1.5 +4%. If the nickel content is less than that given by theformulae, the phase NiAl may be formed in preference to Ni3(Al, Ti) witha consequent great loss in medium and elevated temperature properties.

The element cobalt, which aids in ductility, has been added within therange of 7-16 weight percent. It wouldl the proper balance of elements,the range has been SetV at 7-l6 weight percent. A higher percentagewould bek included at a sacrice of the nickel content thus to form lessNis (Al, Ti) for precipitation hardening; a lower percentage does nothave a signicant effect on ductility. At Al and Ti levels lower thanthat of the alloy of this invention an addition of higher percentages ofcobalt is possible, but the unique characteristics of this alloy islost.

The chromium included in the alloy has its principal eiect as an aid tooxidation resistance. It has been found that a minimum of 8 weightpercent is required to improve the oxidation resistance. However ifchromium is increased to too high a level, for example as high as 20weight percent or more, the chromium is added at lthe expense of nickeland it drops the nickel below the minimum essential inthe above listedformula. ThereforeV it is essential to balance the chromium and cobtaltcontent with the required amount of nickel. Because more cobalt isrequired for ductility purposes, the chromium content is maintained aslow as possible in order to be able to allow the addition of more cobaltwithout detracting trom the nickel content. In addition, high levels ofchromium favor formation of sigma phase which causes great loss ofductility.

It is preferred to have as little iron as possible in the alloy of thisinvention because it does not benefit and can actually detract from theproperties of the alloy. The alloy of this invention can tolerate up toabout 0.5 weight percent Fe in order to allow columbium to be added asferro columbium, a less expensive type of columbiurn addition. Theinclusion of higher percentages of iron, for example above 0.5% willdecrease ductility.

In addition to the careful balance between the elements Ni, Ti and Al toform Ni3(Al, Ti) and the critical relationship between Ni, Co `and Crcontent, a signiticant feature of the alloy of this invention isadjustment of the levels of carbon and the carbide formers to form theproper carbides. The carbide forming elements are specifically selectedto form higher solution temperature phases and to preferentially formcarbides to protect other alloying additions added for other purposes.

From consideration of the carbides, it is required that at least 5.5weight percent Mo be added with the carbon level between E-0.1 weightpercent in order to form MSC type carbide instead of the M23C5 typecarbide. MBC carbide is a high molybdenum carbide stable to highertemperatures and is not dissolved until about 2150 F. The low molybdenumM23C6 carbide is soluble in the 19004950o F. temperature range and wouldbe formed with lower molybdenum and higher chromium. lt is the intentionof the alloy of this invention to form the higher molybdenum, lowerchromium MGC carbide.

Below about 5.5 weight percent Mo the undesirable M23C6 carbide beginsto form. Above about 8 weight percent molybdenum, the molybdenum isadded at the expense of nickel. No real advantage has been found inadding higher molybdenum percentages although some strengthening isachieved from higher percentages of molybdenum in some solutionhardening type of alloys. The `solution hardening value of molybdenumreaches the maximum at about 4-5 weight percent; however it is theintention of this alloy to include molybdenum at higher levels to assurethe formation of MSC carbide. With the previously mentioned Alloy A,MggCG may form (less than 5.5% Mo) the same as in Alloy B shown in thedrawing and having a nominal composition, by weight, of 4.5% Al, 3.5%Ti, 18% Co, 15% Cr, 4.5% Mo, 0.12% C, 0.01% B, balance Ni. Although someprior alloys report a broad molybdenum range, the significance of therange 5.5-8 weight percent Mo in an alloy such as the alloy of thisinvention has been unrecognized heretofore, in view of the relationshipbetween Mo and C in the proper carbon range.

Although it is the intention of the alloy of this invention to causeformation of MSC carbide in preference to M23C6, it is important to keepall the carbides to a relatively low level. Probably more important thanthe lower solution temperature of the M23C6 carbide, is that suchcarbide is to be avoided in the alloy of the type of this inventionbecause M23C6 carbide precipitates in the normal aging and operationranges at the grain boundaries and usually is an embrittler. MGCcar-bide will not precipitate in the grain boundaries excessively. Whenit does, it precipitates in the form of spherical particles which do notharm ductility.

The carbides formed give a small amount of strengthen ing insofar as thehigh temperature stress rupture properties are concerned. They also actto block grain growth in wrought materials. especially the MGC carbide.It has been found that the carbon level must be kept within the range of0.03-01 weight percent particularly with the level of chromium andmolybdenum described above and in view of the carbide former columbiumwhich will be described in more detail later. Above 0.1 weight percentcarbon excessive carbides formed, which in wrought material formcarbides stringers which cause material to split and peel. Anotherreason for keeping the carbon Cil l level below 0.1 weight percent isthat the formation of a large amount of carbide, even though they may betitanium carbide or columbium carbide rather than M26C6, can group inexcessive quantities to form what corresponds to a grain boundarynetwork.

Tungsten has been included in the alloy of this invention within therange of l-3 weight percent because it has been recognized that theproperties of nickel base material can be increased by such an addition.However below 1% no appreciable improvement was recognized. Although theelements W and Mo are sometimes considered equivalent from a solutionhardening point of view, the substitution of l-3 weight percentmolybdenum for the 1 3 weight percent tungsten does not have the sameeffect on the alloy of this invention as does the tungsten inclusion.Molybdenum and tungsten act individually in this case, las would beexpected on a theoretical basis. Over 3 weight percent W does notfurther improve properties, but merely decreases ductility.

Columbium is included in the alloy of this invention, because inaddition to being a solution hardener, it is a more successful carbideformer than titanium. This means that columbium rather than titaniumwill selectively form car-bides with the carbon thus the titanium isfree for the more important action in the Ni3(Al, Ti). Thus the alloy ofthis invention obtains a double benefit from the inclusion of columbium.The inclusion of above 2 weight percent columbium within the range ofthe alloy of this invention begins to embrittle the ralloy by too muchsolution hardening. In addition, columbium may also form an Ni3Cbprecipitation hardening phase which has a lower solution temperaturethan does the Ni3(Al, Ti). Therefore the columbium range for the alloyof this invention is maintained below 2% to avoid displacement of Al andTi from the Ni3(Al, Ti).

Although the element boron has been included in nickel base lalloys ofthis type, a particular and critical minimum amount has been recognized.At least 0.003 weight percent boron must be included in wroughtmaterials to help prevent carbide precipitation in the grain boundary byperforming relatively low melting phases in the grain boundary. Somewhatmore, about 0.006 weight percent boron is required in cast alloys.However, above about 0.4 weight percent there is too great an amount oflow melting phase formed in the grain boundary which results in a weakmaterial.

As representative of the alloy of this invention, a series of alloyswithin the preferred range, by weight, 4.9-6% Al, 2-3% Ti, 13l6% Co,9-l3% Cr, 6-7.2% Mo, 0.04-0.l0% C, 0.2-0.7% Cb, G01-0.02% B, a maximumof .5% Fe, with the balance essentially nickel and irnpurities of Mn,Si, S and Cu were melted and cast into test bars of about 0.250 diametersize to a maximum grain size of about 1/s". The specimens were aged for2-4 hours at 1400 F. and air cooled before testing. The following tableshows the average and maximum tensile data for such preferred form ofthe alloy of this invention.

Tensile Data Ultimate Strength 0.2% Yield Strength (K psi.) (K psi.)Temperature F.)

Avg. Max. Avg. Max.

Through the careful balance of the elements in the alloy of thisinvention, there has been provided a nickel base alloy of improved lowtemperature strength up to about 1700 F. compared with the bestavailable commercial alloy-s. In addition the alloy of this inventionhas at least the same high temperature stress rupture propertiesavailable in alloys of this type.

Although this invention has been described in connection with speciticexamples, it is to be understood that these are exemplary of rather thanlimitations on this invention. It will be understood `by those skilledin the metallurgical arts the modilications and variations of which thisinvention is capable.

What is claimed is:

1. An improved nickel base alloy which inhibits the formation ofembrittling precipitate phases through a balance of carbon, the carbideformers, the solution strengtheners and chromium, consisting essentiallyof, by weight, 4-7% Al; 1.5-4% Ti; 7-16% Co; 8-16% Cr; 5.5-8% Mo;GB-0.1% C; 0.3-1.2% Cb; 0.0030.4% B; 1-3% W up to 0.2% Zr with thebalance nickel and impurities, to form preferentially the MSC typecarbide rather than the M23C5 type carbide, the minimum weightpercentage of nickel when the Al/ Ti ratio is greater than 2 beingcalculated from the formula 4+6 (percent A14-percent Ti) and when theAl/Ti ratio is less than 2 being calculated from the formula 4|5(percent Al+percent Ti).

2. An improved nickel base alloy which inhibits the formation ofembrittling precipitate phases through a balance of carbon, the carbideformers, the solution strengtheners and chromium, consisting essentiallyof, by Weight, 5-6% Al; 2-3% Ti; 13-16% Co; 9-13% Cr; 6-7% Mo; D04-0.10%C; 0.2-0.7% Cb; Q01-0.02% B and 12% W; up to 0.2% Zr with the balancenickel and impurities,

0 to form preferentially the MGC type carbide rather than the MZBCS typecarbide, the minimum Weight percentage of nickel When the Al/Ti ratio isgreater than 2 being calculated from the formula 4-1-6 (percentAl-l-percent Ti) and when the Al/Ti ratio is less than 2 beingcalculated from the formula 44-5 (percent Al-l-percent Ti).

3. An improved nickel base alloy which inhibits the formation ofembrittling precipitate phases through a balance of carbon, the carbideformers, the solution strengtheners and chromium, consisting essentiallyof, by weight, 5.5% Al; 2.5% Ti; 14.5% Co; 11.5% Cr; 6.5% Mo; 0.07% C;0.5% Cb; 0.015% B; 1.5% W with the balance nickel and impurities, toform preferentially the M6() type carbide rather than the M23C6 typecarbide.

References Cited in the le of this patent UNTED STATES PATENTS 2,570,193Bieber et al Oct. 9, 1951 2,570,194 Bieber et al. Oct. 9, 1951 2,712,498Gresham et al July 5, 1955 2,809,110 Darmara Oct. 8, 1957 2,912,323Bieber Nov. 10, 1959 3,107,167 Abkowitz etal Oct. 15, 1963 FOREIGNPATENTS 55,100 France Dec. 20, 1950 737,178 Great Britain Sept. 21, 1955932,273 France Nov. 17, 1947 583,807 Great Britain Dec. 31, 1946 840,496Great Britain July 6, 1960

1. AN IMPROVED NICKEL BASE ALLOY WHICH INHIBITS THE FORMATION OFEMBRITTLING PRECIPITATE PHASES THROUGH A BALANCE OF CARBON, THE CARBIDEFORMERS, THE SOLUTION STRENGTHENERS AND CHROMIUM, CONSISTING ESSENTIALLYOF, BY WEIGHT, 4-7% AL; 1.5-4% TI; 7-16% CO; 8-16% CR; 5.5-8% MO;0.03-0.1% C; 0.3-1.2% CB; 0.003-0.4% B; 1-3% W UP TO 0.2% ZR WITH THEBALANCE NICKEL AND IMPURITIES, TO FORM PREFERENTIALLY THE M5C TYPECARBIDE RATHER THAN THE M23C6 TYPE CARBIDE, THE MINIMUM WEIGHTPERCENTAGE OF NICKEL WHEN THE AL/TI RATIO IS GREATER THAN 2 BEINGCALCULATED FROM THE FORMULA 4+6 (PERCENT AL+PERCENT TI) AND WHEN THEAL/TI RATIO IS LESS THAN 2 BEING CALCULATED FROM THE FORMULA 4+5(PRESENT AL+PERCENT TI).